High quality factor transducers for surface acoustic wave devices

ABSTRACT

The present disclosure relates to acoustic wave devices, and particularly to high quality factor (Q) transducers for surface acoustic wave (SAW) devices. An exemplary SAW device includes an interdigital transducer (IDT) between two reflective gratings to form a resonator. The resonator operates through shear horizontal mode acoustic waves, and therefore suppression of transverse modes (parallel to electrode fingers of the IDT) is desired. A piston mode can be formed in the resonator to suppress transverse modes, which may also increase energy leakage and result in a lower Q. A higher Q is achieved by adding a fast region at an end of one or more of the electrode fingers of the IDT.

FIELD OF THE DISCLOSURE

The present disclosure relates to acoustic wave devices, andparticularly to high quality factor (Q) transducers for surface acousticwave (SAW) devices.

BACKGROUND

Acoustic wave devices are widely used in modern electronics. At a highlevel, acoustic wave devices include a piezoelectric material in contactwith one or more electrodes. Piezoelectric materials acquire a chargewhen compressed, twisted, or distorted, and similarly compress, twist,or distort when a charge is applied to them. Accordingly, when analternating electrical signal is applied to the one or more electrodesin contact with the piezoelectric material, a corresponding mechanicalsignal (i.e., an oscillation or vibration) is transduced therein. Basedon the characteristics of the one or more electrodes on thepiezoelectric material, the properties of the piezoelectric material,and other factors such as the shape of the acoustic wave device andother structures provided on the device, the mechanical signaltransduced in the piezoelectric material exhibits a frequency dependenceon the alternating electrical signal. Acoustic wave devices leveragethis frequency dependence to provide one or more functions.

Surface acoustic wave (SAW) devices, such as SAW resonators and SAWfilters, are used in many applications such as radio frequency (RF)filters. For example, SAW filters are commonly used in second generation(2G), third generation (3G), fourth generation (4G), and fifthgeneration (5G) wireless receiver front ends, duplexers, and receivefilters. The widespread use of SAW filters is due to, at least in part,the fact that SAW filters exhibit low insertion loss with goodrejection, can achieve broad bandwidths, and are a small fraction of thesize of traditional cavity and ceramic filters. As the use of SAWfilters in modern RF communication systems increase, there is a need forSAW filters with improved quality factor (Q) and reduced energy leakage.

SUMMARY

The present disclosure relates to acoustic wave devices, andparticularly to high quality factor (Q) transducers for surface acousticwave (SAW) devices. An exemplary SAW device includes an interdigitaltransducer (IDT) between two reflective gratings to form a resonator.The resonator may operate through shear horizontal mode acoustic waves.The transducer may serve as a waveguide, and therefore suppression oftransverse modes (parallel to electrode fingers of the IDT) is desired.A piston mode can be formed in the resonator to suppress transversemodes, which may also increase energy leakage and result in a lower Q. Ahigher Q is achieved by adding a fast region at an end of one or more ofthe electrode fingers of the IDT.

In one aspect, a SAW device comprises a piezoelectric substrate and anIDT disposed over the piezoelectric substrate. The IDT comprises a firstelectrode and a second electrode. The IDT also comprises a first set ofelectrode fingers coupled to the first electrode, each comprising afirst inside region, a first end defining a first gap with the secondelectrode, and a first fast region at the first end defining a firstpropagation velocity on the piezoelectric substrate which is faster thanthe first inside region. The IDT also comprises a second set ofelectrode fingers interleaved with the first set of electrode fingersand coupled to the second electrode, each comprising a second insideregion and a second end defining a second gap with the first electrode.

In another aspect, a SAW device comprises a piezoelectric substrate andan IDT disposed over the piezoelectric substrate. The IDT comprises afirst electrode and a second electrode. The IDT also comprises a firstset of electrode fingers coupled to the first electrode and having afirst gap from the second electrode at a first end. The IDT alsocomprises a second set of electrode fingers interleaved with the firstset of electrode fingers and coupled to the second electrode, having asecond gap from the first electrode at a second end. The first end ofthe first set of electrode fingers and the second end of the second setof electrode fingers are physically modified to obtain a modifiedvelocity compared to a center region between the first end and thesecond end such that a mode guided in the center region propagates.

Those skilled in the art will appreciate the scope of the presentdisclosure and realize additional aspects thereof after reading thefollowing detailed description of the preferred embodiments inassociation with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the disclosure, andtogether with the description serve to explain the principles of thedisclosure.

FIG. 1A is a schematic diagram of an exemplary surface acoustic wave(SAW) device, a SAW resonator.

FIG. 1B is a schematic diagram of an exemplary ladder type filterincluding the SAW resonator of FIG. 1.

FIG. 1C is a schematic diagram of an exemplary coupled resonator filter(CRF) or double mode SAW (DMS) filter.

FIG. 2A is a schematic diagram of an exemplary piston mode transducer,which may be included in the SAW resonator of FIG. 1A or the CRF of FIG.1C.

FIG. 2B is a schematic diagram of energy leakage in the SAW resonator ofFIG. 1A incorporating the piston mode transducer of FIG. 2A.

FIG. 3A is a schematic diagram of an exemplary SAW device according toaspects of the present disclosure, incorporating an interdigitaltransducer (IDT) with a modified transducer structure.

FIG. 3B is a schematic diagram of another exemplary SAW device,incorporating the IDT with an alternative modification of the transducerstructure.

FIG. 3C is a graphical representation of the main acoustic mode of theexemplary SAW device of FIG. 3A or FIG. 3B in the transverse direction.

FIG. 4A is a schematic diagram of another exemplary SAW device,incorporating the modified transducer structure of FIG. 3A with a pistonmode approach.

FIG. 4B is a schematic diagram of another exemplary SAW device,incorporating the modified transducer structure of FIG. 3B with a pistonmode approach.

FIG. 5A is a schematic diagram of another exemplary SAW device,incorporating the modified transducer structure of FIG. 3A and thepiston mode of FIG. 4A.

FIG. 5B is a graphical representation of an impedance phase of theexemplary SAW device of FIG. 5A.

FIG. 6 is a schematic diagram of another exemplary SAW deviceincorporating the modified transducer structure of FIG. 3B and thepiston mode of FIG. 4B.

FIG. 7A is a top view of an exemplary SAW device implementing the IDT ofFIGS. 3A-6.

FIG. 7B is a cross-section view taken along line A of FIG. 7A.

FIG. 8A is a top view of another exemplary SAW device implementing theIDT of FIGS. 3A-6.

FIG. 8B is a cross-section view taken along line B of FIG. 8A.

FIG. 9A is a top view of another exemplary SAW device implementing theIDT of FIGS. 3A-6.

FIG. 9B is a cross-section view taken along line C of FIG. 9A.

FIG. 10A is a top view of another exemplary SAW device implementing theIDT of FIGS. 3A-6.

FIG. 10B is a cross-section view taken along line D of FIG. 10A.

FIG. 11 is a perspective view of an exemplary SAW device according toaspects of the present disclosure.

FIGS. 12A and 12B are graphical representations of the admittance phaseand Q factor of the exemplary SAW device of FIG. 11.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the embodiments andillustrate the best mode of practicing the embodiments. Upon reading thefollowing description in light of the accompanying drawing figures,those skilled in the art will understand the concepts of the disclosureand will recognize applications of these concepts not particularlyaddressed herein. It should be understood that these concepts andapplications fall within the scope of the disclosure and theaccompanying claims.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present disclosure. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It will be understood that when an element such as a layer, region, orsubstrate is referred to as being “on” or extending “onto” anotherelement, it can be directly on or extend directly onto the other elementor intervening elements may also be present. In contrast, when anelement is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present.Likewise, it will be understood that when an element such as a layer,region, or substrate is referred to as being “over” or extending “over”another element, it can be directly over or extend directly over theother element or intervening elements may also be present. In contrast,when an element is referred to as being “directly over” or extending“directly over” another element, there are no intervening elementspresent. It will also be understood that when an element is referred toas being “connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

Relative terms such as “below” or “above” or “upper” or “lower” or“horizontal” or “vertical” may be used herein to describe a relationshipof one element, layer, or region to another element, layer, or region asillustrated in the Figures. It will be understood that these terms andthose discussed above are intended to encompass different orientationsof the device in addition to the orientation depicted in the Figures.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes,” and/or “including” when used herein specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms used herein should be interpreted ashaving a meaning that is consistent with their meaning in the context ofthis specification and the relevant art and will not be interpreted inan idealized or overly formal sense unless expressly so defined herein.

The present disclosure relates to acoustic wave devices, andparticularly to high quality factor (Q) transducers for surface acousticwave (SAW) devices. An exemplary SAW device includes an interdigitaltransducer (IDT) between two reflective gratings to form a resonator.The resonator may operate through shear horizontal mode acoustic waves.The transducer may serve as a waveguide, and therefore suppression oftransverse modes (parallel to electrode fingers of the IDT) is desired.A piston mode can be formed in the resonator to suppress transversemodes, which may also increase energy leakage and result in a lower Q. Ahigher Q is achieved by adding a fast region at an end of one or more ofthe electrode fingers of the IDT.

To assist in understanding aspects of the present disclosure, anoverview of SAW devices is provided with reference to FIGS. 1A-2B.Exemplary SAW devices incorporating a modified IDT to improve Q factorare illustrated in FIGS. 3A-6. Top and cross-section views of exemplarySAW devices incorporating the modified IDT are illustrated in FIGS.7A-10B. A perspective view of an exemplary SAW device according toaspects of the present disclosure is illustrated in FIG. 11. Performanceof the exemplary SAW device of FIG. 11 is illustrated in FIGS. 12A and12B.

FIG. 1A is a schematic diagram of an exemplary SAW device, a SAWresonator 10. The SAW resonator 10 includes a transducer 12 (which maybe an IDT) to convert an electric signal to an acoustic wave propagatingon a surface of a piezoelectric substrate 14. The transducer 12 ispositioned between two reflective gratings 16 to form the SAW resonator10. The SAW resonator 10 can be incorporated into a device such as afilter or duplexer by interconnecting several SAW resonators 10 (and/orother resonators) electrically and/or acoustically.

FIG. 1B is a schematic diagram of an exemplary ladder type filter 18including the SAW resonator 10 of FIG. 1A. In this regard, several SAWresonators 10 are interconnected in an electrical network to form theexemplary ladder type filter 18. In other examples, the SAW resonators10 can be arranged in other ways, such as a lattice configuration.

In still other examples, the SAW resonators 10 can be coupledacoustically, as depicted in FIG. 1C. FIG. 1C is a schematic diagram ofan exemplary coupled resonator filter (CRF) 20 or double mode SAW (DMS)filter. In the CRF 20, several transducers 12 are positioned between thereflective gratings 16. One or several transducers 12 are connected inparallel between an input signal IN and a reference (e.g., ground),while another one or more transducers 12 are connected between thereference and an output signal OUT. The electrical signal at the inputis transformed into an acoustical signal on the surface of thepiezoelectric substrate 14. This acoustic wave propagates between inputand output transducers 12 and is converted back into an electricalsignal. Through selection of the number of transducers 12, their sizes,frequencies, apertures and other parameters, a band pass filter or othertype of filter can be produced. In some examples, several CRF 20 stagesare cascaded inside a filter to improve its performance. In someexamples, some of the transducers 12 are not connected to a voltage andthe reference voltage (ground) but are instead connected between twovoltages which may be equal in amplitude and with 180 degrees phasedifference (e.g., a differential or balanced drive). In some examples,the reflective gratings 16 may be absent or alternatively reflectivegratings 16 may be inserted between the transducers 12.

A radio frequency (RF) filter may incorporate the SAW resonator 10 ofFIG. 1A or the CRF 20 of FIG. 1C, in which one or more transducers 12are disposed between the reflective gratings 16 over the piezoelectricsubstrate 14. Any appropriate material may be used for the piezoelectricsubstrate 14, such as lithium niobate or lithium tantalate. To improvethe temperature coefficient of frequency (TCF) of a SAW resonator 10 orCRF 20, when lithium niobate (which has a negative TCF) is usedelectrodes of the transducers 12 can be embedded within a dielectricwith a positive TCF (e.g., silicon oxide). However, because the TCF of alithium tantalate piezoelectric substrate 14 is closer to 0, theelectrodes may not be embedded in a dielectric. In addition, in caseswhere the electrodes are not embedded in the dielectric, one or moredielectric films may be deposited on top of the electrodes (e.g., toserve as passivation layers). In some cases, it may be furtheradvantageous to add a dielectric layer between the electrodes and thepiezoelectric substrate (e.g., to reduce the coupling factor).

The Q factor of the SAW resonator 10 or CRF 20 is linked to electricaland acoustical losses. The sources of acoustical losses are the viscouslosses in the materials and the losses due to acoustic leakage. If theSAW resonator 10 or CRF 20 is not designed properly, a significant partof the acoustic energy can leak from the transducers 12. This leakagemay be a bulk acoustic wave (BAW) leakage (meaning that the acousticenergy is leaking inside the piezoelectric substrate 14) or a transverseleakage (meaning that the acoustic energy is leaking on the surfaceoutside of an active aperture of the transducers 12). Transverse leakagecan happen on both niobate and tantalate substrates, while in usualsituations BAW leakage happens mostly on lithium tantalate substrates.

To suppress the transverse leakage, the SAW resonator 10 or CRF 20 canbe designed as a waveguide in the transverse direction. If a slownesscurve of the surface acoustic wave in the transducer 12 has a convexshape, guiding is obtained when peripheral regions of the transducer 12have a faster velocity than inside regions, resulting in an evanescentwave in the peripheral regions. If the slowness curve has a concaveshape, guiding is obtained if the peripheral regions have a slowervelocity than the inside regions. In most situations, the slowness curvein the transducer 12 has a convex shape, and embodiments of thisdisclosure are generally described with respect to this situation. Onsome substrates (e.g., lithium tantalate), the shape of the slownesscurve is concave for the free surface, but it becomes convex in thetransducer 12 due to the electrode presence when the metal thicknessincreases. To use this guiding effect, for example, each side of theactive aperture can be provided with a region with a faster velocity,with the region being wide enough to avoid leakage. The fast region canbe a gap at an end (e.g., periphery) of the interdigital electrodes. Byusing a gap larger than about one wavelength, the acoustic energy can beconfined inside the acoustic aperture and improve the Q factor. However,acoustic waves are reflected at the edge of the transducers 12, givingrise to transverse modes. To suppress the transverse modes, it ispossible to add a slow velocity region at the edge of the active region.This allows the propagation of a piston mode in the transducer 12. Thepiston mode has an amplitude which is essentially flat inside theaperture and which decreases in the slow edge region and in the fastgap. This mode amplitude is matched to the excitation shape with theresult that almost no other mode is excited.

FIG. 2A is a schematic diagram of an exemplary piston mode transducer22, which may be included in the SAW resonator 10 of FIG. 1A or the CRF20 of FIG. 1C. In this regard, a slow velocity in an edge of the pistonmode transducer 22 can be created by using a larger electrode width inan edge region 24 than in a center region 26 to obtain a slowervelocity. In this regard, FIG. 2A includes a velocity diagram 28indicating relative velocities of positions of the piston modetransducer 22. These velocities and an appropriate choice of the slowregion width result in a piston-shaped acoustic mode 30 of the pistonmode transducer 22, having an amplitude which is essentially flat insidethe aperture (e.g., the amplitude may be within 10% of a flat curve inthe aperture) and which decreases in the slow edge region and in thefast gap. This may further reduce a coupling to spurious modes in theaperture. If the gap between the electrodes is wide enough, the acousticwave amplitude in the bussbars may be negligible and the velocity in thebussbars may not significantly impact performance of the SAW resonator10.

FIG. 2B is a schematic diagram of energy leakage in the SAW resonator 10of FIG. 1A incorporating the piston mode transducer 22 of FIG. 2A. Ahigher Q can be obtained for the SAW resonator 10 by having apolarization of the acoustic waves mostly along a desired direction ofpropagation (e.g., across the interdigital electrodes) and/orvertically. For example, on lithium niobate near to 128°, the pistonmode transducer 22 can induce a quasi-Rayleigh mode in which themechanical displacements are mostly in the sagittal plane (i.e., theplane formed by the propagation direction and the vertical axis) anddisplacements in the transverse direction (e.g., parallel to theinterdigital electrodes) are very small. However, in some cases theexcited mode of the piston mode transducer 22 is a shear horizontalmode, such as for lithium tantalate or lithium niobate with anorientation between Y−30° and Y+60°. In the shear horizontal mode, thedirection of the acoustical displacements is mainly parallel to theelectrodes and the piston mode transducer 22 suffers from a severedrawback.

When the displacement in the transverse direction is large, the edges ofthe interdigital electrodes (in the transverse directions) move in phaseand generate acoustic waves 32 parallel to the interdigital electrodes,as shown in FIG. 2B. Oblique shear horizontal waves 34 and obliqueRayleigh waves 36 may also be generated. The phenomenon of generatingthe acoustic waves 32 parallel to the electrodes may be present in othercases, but it is larger in the case of a shear horizontal mode. Inaddition, the large gap of the piston mode transducer 22 can make theacoustic waves 32 stronger, resulting in lower Q. To reduce or avoidthis acoustic radiation, embodiments described herein modify thetransducer structure in the transverse direction to avoid having asignificant acoustic energy close to the interdigital electrode end gap.

FIG. 3A is a schematic diagram of an exemplary SAW device 40 accordingto aspects of the present disclosure, incorporating an IDT 42 with amodified transducer structure. In this regard, the SAW device 40 mayinclude or be incorporated in a SAW resonator 10 as described above withrespect to FIG. 1A, an RF filter (e.g., the ladder type filter 18) asdescribed above with respect to FIG. 1B, and/or a CRF 20 as describedabove with respect to FIG. 1C. In addition, the IDT 42 may form thetransducer 12 of FIG. 1A.

In an exemplary aspect, the SAW device 40 includes a piezoelectricsubstrate 44 (e.g., incorporated in the piezoelectric substrate 14) andthe IDT 42 disposed over the piezoelectric substrate 44. The IDT 42includes a first electrode 46 and a second electrode 48, which may beparallel bussbars coupled to input and output signals respectively. Afirst set of electrode fingers 50 is coupled to the first electrode 46,and a second set of electrode fingers 52 is interleaved with the firstset of electrode fingers 50 and coupled to the second electrode 48. Atleast one of the electrode fingers (e.g., from the first set ofelectrode fingers 50 and/or the second set of electrode fingers 52) ismodified to reduce acoustic wave amplitude at a periphery of theelectrode fingers 50, 52. This is done by changing the velocity in orderto make the wave evanescent at an edge of the SAW device 40 (e.g.,around ends of the electrode fingers 50, 52). This, in turn, reducesacoustic leakage of the SAW device 40.

In some cases, the piezoelectric substrate 44 of the SAW device 40 canbe lithium tantalate (though this is not required, and other materialsmay be used, such as lithium niobate or other piezoelectricmaterial(s)). In at least the case of lithium tantalate, the acousticalmode is a shear horizontal mode. The surface wave velocity can be largerthan the velocity of the shear vertical bulk mode and a portion of theenergy in the SAW device 40 is radiated in the volume of thepiezoelectric substrate 44. The result for the surface wave isequivalent to propagation losses, and may be referred to as a “leakySAW.”

Two approaches may suppress these bulk radiation losses. The firstapproach replaces the piezoelectric substrate 44 of lithium tantalatewith a thin layer of lithium tantalate bonded on a faster velocitysubstrate (e.g., silicon). This approach guides the energy inside thelayer of lithium tantalate since the velocity in the supportingsubstrate is larger than the velocity of the surface wave. The secondapproach increases the metal thickness of the electrode fingers 50, 52enough to reduce the surface wave velocity below the bulk wavevelocities. This also results in the suppression of the bulk moderadiation. In such devices, it is found that transverse modes aredifficult to suppress using a standard piston mode design. In addition,these approaches may add losses to the SAW device 40 and complicate thestructure of the transverse mode.

Aspects of the present disclosure modify the structure of the SAW device40 with a shear horizontal acoustic mode to suppress electroacousticcoupling to the Rayleigh mode. While acoustic coupling may still existbetween the Rayleigh mode and the shear horizontal mode, the IDT 42 isdesigned to obtain a shear horizontal mode at a velocity which is lowerthan the Rayleigh mode velocity. In this manner, the Rayleigh modecannot be excited at the frequency of the SAW device 40.

As shown in FIG. 3A, a fast region is added at an end of one or moreelectrode fingers of the IDT 42. For example, each of the first set ofelectrode fingers 50 includes a first inside region 54 and a first end56 defining a first gap 58 with the second electrode 48. A first fastregion 60 at the first end 56 defines a first propagation velocity atthe surface of the piezoelectric substrate 44 which is faster than thefirst inside region 54. Similarly, each of the second set of electrodefingers 52 includes a second inside region 62 and a second end 64defining a second gap 66 with the first electrode 46. A second fastregion 68 at the second end 64 defines a second propagation velocity(which may be the same as the first propagation velocity) at the surfaceof the piezoelectric substrate 44 which is faster than the second insideregion 62.

Each of the first fast region 60 and the second fast region 68 are largeenough to avoid having a significant portion of the acoustic energyreach the first end 56 of the first set of electrode fingers 50 or thesecond end 64 of the second set of electrode fingers 52. In this manner,the shear mode may not be present at the first gap 58 or the second gap66, and radiation of the Rayleigh mode parallel to the first set ofelectrode fingers 50 and the second set of electrode fingers 52 issuppressed (e.g., reduced or eliminated). The acoustic mode of the IDT42 is thus guided between the first fast region 60 and the second fastregion 68.

FIG. 3B is a schematic diagram of another exemplary SAW device 40,incorporating the IDT 42 with an alternative modification of thetransducer structure. In FIG. 3A, the first end 56 of each of the firstset of electrode fingers 50 is modified to include the first fast region60, and the second end 64 of each of the second set of electrode fingers52 is modified to include the second fast region 68. Alternatively, asillustrated in the exemplary SAW device 40 of FIG. 3B, it may bebeneficial to modify a base region of both sets of electrode fingers 50,52 such that the first fast region 60 and the second fast region 68 aredefined through all the electrode fingers.

In this regard, each of the first set of electrode fingers 50 includes afirst base region 70, and the first inside region 54 is between thefirst end 56 and the first base region 70. The first set of electrodefingers 50 also includes the first fast region 60 at the first end 56and the second fast region 68 between the first base region 70 and thefirst inside region 54. In addition, each of the second set of electrodefingers 52 includes a second base region 72, and the second insideregion 62 is between the second end 64 and the second base region 72.The second set of electrode fingers 52 also includes the second fastregion 68 at the second end 64 and the first fast region 60 between thesecond base region 72 and the second inside region 62.

Thus, the first fast region 60 defines a first propagation velocity onthe piezoelectric substrate 44 which is faster than the first insideregion 54 and the second inside region 62. The second fast region 68 atthe second end 64 defines a second propagation velocity (which may bethe same as the first propagation velocity) on the piezoelectricsubstrate 44 which is faster than the first inside region 54 and thesecond inside region 62.

FIG. 3C is a graphical representation of a main acoustic mode 74 of theexemplary SAW device 40 of FIG. 3A or FIG. 3B in the transversedirection. As illustrated in FIG. 3C, the main acoustic mode 74 of theIDT 42 is guided between the first fast region 60 and the second fastregion 68. The main acoustic mode 74 has a cosine shape in this activeregion and is evanescent in the first fast region 60 and the second fastregion 68. At the first gap 58 and the second gap 66, the amplitude ofthe main acoustic mode 74 is at or near zero, suppressing generation ofleaking Rayleigh modes and increasing the Q factor of the IDT 42. Inthis manner, each of the first set of electrode fingers 50 and thesecond set of electrode fingers 52 are physically modified to obtain amodified velocity (e.g., a faster velocity) in the first fast region 60and the second fast region 68 compared to a center region between thefirst end 56 and the second end 64 such that a mode guided in the centerregion propagates. In some examples, the first inside region 54 of thefirst set of finger electrodes 50 and the second inside region 62 of thesecond set of finger electrodes 52 define the center region.

FIG. 4A is a schematic diagram of another exemplary SAW device 40,incorporating the modified transducer structure of FIG. 3A with a pistonmode approach. The IDT 42 of FIG. 3A may experience transverse modes. Toavoid this, the IDT 42 of FIG. 4A incorporates a piston mode approach byfurther including a slower region between the fast region 60, 68 of eachelectrode finger and the active region (e.g., the inside regions 54,62). In this regard, each of the first set of electrode fingers 50 alsoincludes a first slow region 76 between the first inside region 54 andthe first fast region 60, having a propagation velocity lower than thefirst inside region 54. In some examples, each of the first set ofelectrode fingers 50 may also include a second slow region 78 betweenthe first inside region 54 and the first base region 70. By includingthe first fast region 60, the IDT 42 can also have a smaller first gap58 than the piston mode transducer 22 of FIG. 2A (e.g., less than orequal to an electrode width of the first inside region 54).

Similarly, each of the second set of electrode fingers 52 also includesthe second slow region 78 between the second inside region 62 and thesecond fast region 68, having a propagation velocity lower than thesecond inside region 62. In some examples, each of the second set ofelectrode fingers 52 may include the first slow region 76 between thesecond inside region 62 and the second base region 72. Thus, similar tothe piston mode transducer 22 of FIG. 2A, for the IDT 42 of FIG. 4Athese velocities and a proper choice of width for the slow regions 76,78 result in a piston-shaped acoustic mode 80 having a flat amplitude inthe center region (e.g., defined by the first inside region 54 and thesecond inside region 62). In some examples, the amplitude of theacoustic mode 80 is within 10% of a flat curve in the center region. Inaddition, coupling of spurious modes may be reduced in the centerregion. The piston-shaped acoustic mode 80 is also evanescent in thefirst fast region 60 and the second fast region 68, and has a sinusoidalshape in the first slow region 76 and the second slow region 78.

The different velocities in each of the first set of electrode fingers50 and the second set of electrode fingers 52 can be obtained through anumber of techniques. For example, an electrode width of the first fastregion 60 (and/or the second fast region 68), defined transverse to thefirst gap 58, can be smaller than an electrode width of the first insideregion 54 (or the second inside region 62, respectively). Conversely, anelectrode width of the first slow region 76 and the second slow region78 can be larger than the electrode width of the first inside region 54(or the second inside region 62, respectively). In other examples,instead of varying the width of the electrode fingers, the metalthickness can be different in each region. In this case, the first slowregion 76 and the second slow region 78 have more metal than the firstinside region 54 and the second inside region 62. The first fast region60 and the second fast region 68 have less metal than the first insideregion 54 and the second inside region 62. Other cases may vary thematerials of the IDT 42, and multiple such velocity control approachescan be combined. Additional velocity control approaches are furtherdescribed below with respect to FIGS. 5A-11.

FIG. 4B is a schematic diagram of another exemplary SAW device 40,incorporating the modified transducer structure of FIG. 3B with a pistonmode approach. In FIG. 4A, the first end 56 of each of the first set ofelectrode fingers 50 is modified to include the first fast region 60,and the second end 64 of each of the second set of electrode fingers 52is modified to include the second fast region 68. Alternatively, asillustrated in the exemplary SAW device 40 of FIG. 4B, it may bebeneficial to modify a base region of both sets of electrode fingers 50,52 such that the first fast region 60 and the second fast region 68 aredefined through all the electrode fingers.

In this regard, each of the first set of electrode fingers 50 includesthe first fast region 60 at the first end 56 and the second fast region68 between the first base region 70 and the first inside region 54. Eachof the first set of electrode fingers 50 also includes the first slowregion 76 between the first inside region 54 and the first fast region60 and the second slow region 78 between the first inside region 54 andthe second fast region 68.

In addition, each of the second set of electrode fingers 52 includes thesecond fast region 68 at the second end 64 and the first fast region 60between the second base region 72 and the second inside region 62. Eachof the second set of electrode fingers 52 also includes the second slowregion 78 between the second inside region 62 and the second fast region68 and the first slow region 76 between the second inside region 62 andthe first fast region 60.

FIG. 5A is a schematic diagram of another exemplary SAW device 40,incorporating the modified transducer structure of FIG. 3A and thepiston mode of FIG. 4A. The exemplary SAW device 40 of FIG. 5A includesan IDT 42, illustrated showing only a first electrode finger 50 coupledto the first electrode 46 and a second electrode finger 52 coupled tothe second electrode 48, but generally includes additional interleavedelectrode fingers. For example, a transducer can be realized byrepeating these two electrode fingers 50, 52 several times, typicallybetween 10 and 100. In an exemplary aspect, the IDT 42 is made on alithium tantalate substrate with an orientation Y+13°. The IDT 42 isformed with platinum having a relative thickness to a desired wavelengthof 10% and copper with a thickness relative to the desired wavelength of5.4%. The wavelength is defined herein as twice a distance betweencenters of two consecutive electrode fingers 50, 52.

The first inside region 54, the second inside region 62, the first slowregion 76, and the second slow region 78 have a 50% duty factor wherethe duty factor is the ratio of the electrode width to the periodbetween electrodes. The first fast region 60, the second fast region 68,the first base region 70, and the second base region 72 have a 35% dutyfactor. A width of the first fast region 60 and the second fast region68 in the transverse direction (e.g., parallel to the first gap 58) is108% of the wavelength. Since the IDT 42 is optimized to reduce theacoustic energy close to the first gap 58 and the second gap 66, thesegaps 58, 66 can be reduced. In the example depicted in FIG. 5A, a dummyelectrode 82 can be included opposite the respective gap 58, 66, thoughthis is not required. The velocity in the first slow region 76 and thesecond slow region 78 can be reduced by adding an additional layer ofmetal (e.g., copper). In this case, the thickness of the additionallayer is 1.9% of the wavelength. The slow region width in the transversedirection is 75% of the wavelength. Both the width of the slow region76, 78 and its additional thicknesses are chosen to reduce the responseof the transverse modes as much as possible. This optimum is normallyobtained when a mode flat in the aperture (e.g., piston mode) propagatesin the SAW device 40.

FIG. 5B is a graphical representation of an impedance phase of theexemplary SAW device 40 of FIG. 5A. In this regard, the SAW device 40has an impedance phase of 90 degrees around the desired frequency of0.58 gigahertz (GHz) (e.g., between the resonance frequency of about0.565 GHz and the antiresonance frequency of about 0.59 GHz) and almostno spurious modes.

FIG. 6 is a schematic diagram of another exemplary SAW device 40,incorporating the modified transducer structure of FIG. 3B and thepiston mode of FIG. 4B. The SAW device 40 of FIG. 6 obtains the velocitydifferences described above by adapting the materials (e.g., metals)used in the different regions. For example, the first inside region 54and the second inside region 62 (and in some cases the first base region70 and second base region 72) can have a platinum thickness of 10% ofthe desired wavelength and a copper thickness of 5.4% of the desiredwavelength. In the first slow region 76 and the second slow region 78,the copper thickness is increased to about 7.9% of the desiredwavelength. In the first fast region 60 and the second fast region 68,the copper thickness is reduced to 3.5% of the desired wavelength.Alternatively, the platinum thickness can be varied in all or some ofthe regions. In the case of the electrode fingers 50, 52 being embeddedin a dielectric (e.g., silicon oxide), the first slow region 76 and thesecond slow region 78 can be made by adding a strip of metal ordielectric and the fast region by changing the dielectric thickness oradding a layer of silicon nitride or aluminum nitride.

As with FIG. 5A, the SAW device 40 of FIG. 6 is shown with only thefirst finger electrode 50 and the second finger electrode 52, but theseare generally repeated several times (e.g., 10 to 100 times). Inaddition, repeated periods (e.g., pairs of the first finger electrode 50and the second finger electrode 52) can be modified versions of the baseperiod to facilitate improved filter design. For example, the period canvary or the finger electrodes 50, 52 can be connected to differentelectrical ports (for example in CRFs as in FIG. 1C), while maintainingthe velocity profile described above. Example layouts of differentvelocity control approaches are further described below with respect toFIGS. 7A-10B.

FIG. 7A is a top view of an exemplary SAW device 40 implementing the IDT42 of FIGS. 3A-6. FIG. 7B is a cross-section view taken along line A ofFIG. 7A. In this regard, electrodes 84 of the IDT 42 are deposited ontothe piezoelectric substrate 44 and embedded in a dielectric layer 86(e.g., silicon oxide or another appropriate dielectric). A passivationlayer 88 can be disposed over the dielectric layer 86, though this isnot required. The passivation layer 88 can include, for example, atleast one of silicon nitride, aluminum nitride, aluminum oxide, ordiamond.

For each of the first set of electrode fingers 50, the first slow region76 and the second slow region 78 are obtained by embedding a slowmaterial 90 (e.g., a metallic layer or other slow material) inside thedielectric layer 86. In other examples, the slow material 90 can beplaced directly on top of the electrodes 84 (if the slow material 90 isnot conductive), on top of the dielectric layer 86 or the passivationlayer 88. In addition, the first fast region 60 is generated by reducingthe metal thickness of the electrode 84. It should be understood thatwhile FIGS. 7A and 7B are described with respect to the first set ofelectrode fingers 50, each of the second set of electrode fingers 52 canbe similarly formed. This applies with respect to FIGS. 8A-10B as well.

FIG. 8A is a top view of another exemplary SAW device 40 implementingthe IDT 42 of FIGS. 3A-6. FIG. 8B is a cross-section view taken alongline B of FIG. 8A. Similar to FIGS. 7A and 7B, the electrodes 84 of theIDT 42 are deposited onto the piezoelectric substrate 44 and embedded inthe dielectric layer 86. For each of the first set of electrode fingers50, the first fast region 60 is instead obtained by adding a fastmaterial over the dielectric layer 86. For example, a thickness of thepassivation layer 88 may be increased over the first fast region 60compared with the first inside region 54, the first slow region 76, andthe second slow region 78 (e.g., by depositing additional siliconnitride over the first fast region 60 and/or partially etching thepassivation layer 88 in the first inside region 54).

FIG. 9A is a top view of another exemplary SAW device 40 implementingthe IDT 42 of FIGS. 3A-6. FIG. 9B is a cross-section view taken alongline C of FIG. 9A. Similar to FIGS. 7A-8B, the electrodes 84 of the IDT42 are deposited onto the piezoelectric substrate 44 and embedded in thedielectric layer 86. For each of the first set of electrode fingers 50,the velocities of the various regions are obtained by modulating theamount of fast material over the dielectric layer 86. For example, thepassivation layer 88 has a first thickness over the first slow region 76and the second slow region 78 and a second thickness greater than thefirst thickness over the first inside region 54. The passivation layer88 has a third thickness greater than the first or second thickness overthe first fast region 60. As the passivation layer 88 grows thicker, thevelocity increases.

A potential drawback for the SAW devices 40 described with respect toFIGS. 7A-9B can be a loss of coupling factor. The main acoustical modeof the IDT 42 is evanescent in the first fast region 60 and the secondfast region 68. Since the fast regions 60, 68 are part of the IDT 42,they have a capacitance but are not useful to generate acoustic energy.This loss of coupling factor can be addressed by reducing thecapacitance in the fast regions 60, 68 by inserting a dielectric layerbetween the electrodes 84 and the piezoelectric substrate 44 in theseregions. Another potential drawback is that a spurious mode can beexcited in the first end 56 and second end 64 (see FIGS. 3A and 4A). Byadding a dielectric layer between the electrode 84 and the piezoelectricsubstrate 44 in the fast regions 60, 68, the electroacoustic coupling isdrastically reduced in the fast edge region. An example is shown inFIGS. 10A and 10B.

FIG. 10A is a top view of another exemplary SAW device 40 implementingthe IDT 42 of FIGS. 3A-6. FIG. 10B is a cross-section view taken alongline D of FIG. 10A. Unlike FIGS. 7A-9B, the electrodes 84 of the IDT 42are not embedded in the dielectric layer 86. However, a dielectric layer92 is inserted between the electrode 84 and the piezoelectric substrate44 in the first fast region 60 (and/or the first base region 70). Thefirst slow region 76 and the second slow region 78 are formed by addingsome material to the electrodes 84. In the case of FIGS. 10A and 10B,the dielectric layer 92 is a fast material and makes the first fastregion 60. If a different dielectric material is used, the velocity inthe first fast region 60 can be modulated by changing (e.g., reducing)the metal thickness of the electrode 84 or the electrode 84 width. WhileFIGS. 10A and 10B are illustrated with respect to an IDT 42 not embeddedin the dielectric layer 86 of FIGS. 7A-9B, in other examples thedielectric layer 86 may be included. In this case, the first slow region76 and the second slow region 78 can instead be generated as describedin FIGS. 7A-9B, such as by using the slow material 90 in the dielectriclayer 86 or on top of the dielectric layer 86, or by modulating a fastmaterial on top of the dielectric layer 86.

FIG. 11 is a perspective view of an exemplary SAW device 40 according toaspects of the present disclosure. The SAW device 40 may be formed in amanner such as described above with respect to FIGS. 3A-10B. In anexemplary aspect, the piezoelectric substrate 44 is lithium tantalatewith orientation of Y+10°. The electrode period is 2 microns (μm). Theelectrodes 84 are formed of tungsten 94 with a thickness of 6000angstroms (Å) and aluminum 96 with a thickness of 2000 Å. The electrodes84 are not embedded in a dielectric layer.

This physical structure of the IDT 42 is modulated to reduce thecoupling of the Rayleigh mode. Thus, the first fast region 60 is made byusing a 2000 Å layer of aluminum nitride 98 between the electrodes 84and the piezoelectric substrate 44. The first slow region 76 is made inthis case by increasing the duty factor to 60%.

The IDT 42 of FIG. 11 can be used on a traditional piezoelectricsubstrate 44, but can also be used on a substrate made of a thinpiezoelectric film bonded (or deposited) on top of a support substrate.Additional layers can be inserted between the piezoelectric film and thesupporting substrate. In some cases, the electrodes 84 may be embeddedinside a dielectric material (e.g., dielectric layer 86 of FIGS. 7A-9B).

FIGS. 12A and 12B are graphical representations of the admittance phaseand Q factor of the exemplary SAW device 40 of FIG. 11. FIG. 12Aillustrates an admittance phase 100, with better than 85° admittancephase at the desired frequency of 0.54 GHz (e.g., between 0.53 GHz and0.5525 GHz). FIG. 12B illustrates a Bode plot 102 of the Q factor, witha Q factor of better than 4000 at the desired frequency of 0.54 GHz(e.g., between 0.5375 GHz and 0.541 GHz).

Those skilled in the art will recognize improvements and modificationsto the preferred embodiments of the present disclosure. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

What is claimed is:
 1. A surface acoustic wave (SAW) device, comprising:a piezoelectric substrate; and an interdigital transducer (IDT) disposedover the piezoelectric substrate, comprising: a first electrode; asecond electrode; a first set of electrode fingers comprising a firstmetal and a second metal and coupled to the first electrode, eachcomprising: a first inside region; a first end defining a first gap withthe second electrode; a first fast region adjacent the first end havinga reduced thickness of the first metal relative to the first insideregion to define a first propagation velocity on the piezoelectricsubstrate which is faster than the first inside region; and a first slowregion between the first inside region and the first fast region havingan increased thickness of the first metal relative to the first insideregion to define a second propagation velocity through the piezoelectricsubstrate which is slower than the first inside region; and a second setof electrode fingers interleaved with the first set of electrode fingersand coupled to the second electrode, each comprising: a second insideregion; and a second end defining a second gap with the first electrode.2. The SAW device of claim 1, wherein acoustic waves are guided in thefirst inside region and the second inside region.
 3. The SAW device ofclaim 1, wherein an electrode width of the first fast region, definedtransverse to the first gap, is smaller than an electrode width of thefirst inside region.
 4. The SAW device of claim 1, wherein a metalthickness of the first fast region over the piezoelectric substrate issmaller than a metal thickness of the first inside region.
 5. The SAWdevice of claim 1, wherein a material of the first fast region differsfrom a material of the first inside region and produces the firstpropagation velocity.
 6. The SAW device of claim 1, further comprising adielectric layer over the piezoelectric substrate and the IDT.
 7. TheSAW device of claim 6, further comprising a passivation layer over thedielectric layer having a greater thickness over the first fast region.8. The SAW device of claim 7, wherein the passivation layer comprises atleast one material selected from the group of aluminum nitride, aluminumoxide, and diamond.
 9. The SAW device of claim 1, wherein a width of thefirst fast region, defined parallel to the first gap, is at least onewavelength.
 10. The SAW device of claim 1, wherein each of the secondset of electrode fingers further comprises a second fast region adjacentthe second end defining a third propagation velocity through thepiezoelectric substrate which is faster than the second inside region.11. The SAW device of claim 10, wherein the first set of electrodefingers and the second set of electrode fingers define a mode within 10%of a flat curve in the first inside region.
 12. The SAW device of claim11, wherein each of the second set of electrode fingers furthercomprises a second slow region between the second inside region and thesecond fast region which defines a fourth propagation velocity throughthe piezoelectric substrate which is slower than the second insideregion.
 13. The SAW device of claim 12, wherein an electrode width ofthe first slow region and the second slow region, defined transverse tothe first gap and the second gap, is larger than an electrode width ofthe first inside region.
 14. The SAW device of claim 12, wherein athickness of the first slow region and the second slow region over thepiezoelectric substrate is larger than a thickness of the first insideregion.
 15. The SAW device of claim 12, wherein a dielectric layer isdeposited over the first slow region and the second slow region.
 16. TheSAW device of claim 12, further comprising a dielectric layer over thepiezoelectric substrate and the IDT; wherein a metallic layer isembedded within the dielectric layer over the first slow region and thesecond slow region.
 17. The SAW device of claim 1, further comprising adielectric layer between the piezoelectric substrate and the first setof electrode fingers in the first fast region.
 18. The SAW device ofclaim 17, wherein the dielectric layer is a fast layer comprising atleast one of silicon nitride or aluminum nitride.
 19. The SAW device ofclaim 1, wherein the piezoelectric substrate comprises at least one oflithium tantalate or lithium niobate.
 20. The SAW device of claim 1,wherein a third propagation velocity in the first gap is faster than thefirst fast region.
 21. A surface acoustic wave (SAW) device, comprising:a piezoelectric substrate; and an interdigital transducer (IDT) disposedover the piezoelectric substrate, comprising: a first electrode; asecond electrode; a first set of electrode fingers coupled to the firstelectrode and having a first gap from the second electrode at a firstend; and a second set of electrode fingers interleaved with the firstset of electrode fingers and coupled to the second electrode, having asecond gap from the first electrode at a second end; wherein: the firstset of electrode fingers and the second set of electrode fingerscomprise a first metal and a second metal; a thickness of the firstmetal in the first end of the first set of electrode fingers and thesecond end of the second set of electrode fingers is reduced to obtain afast region with a faster velocity compared to a center region betweenthe first end and the second end; and the thickness of the first metalin a slow region between the fast region and the center region isincreased to have a slower velocity than the center region.
 22. The SAWdevice of claim 21, wherein gap regions defined by the first gap and thesecond gap have a faster velocity than the first end and the second end.23. The SAW device of claim 22, wherein the first end and the second endare physically modified to have a smaller width, defined transverse tothe first gap and the second gap, than the center region.
 24. The SAWdevice of claim 22, wherein the first end and the second end arephysically modified to have a smaller thickness over the piezoelectricsubstrate than the center region.
 25. The SAW device of claim 21,wherein the center region is further physically modified to reduce acoupling to spurious modes in the center region.